border-none()
  &:after
    height 0px

//实现一物理像素的下边框
one-px($color)
  position relative
  &:after
    position absolute
    bottom 0
    display block
    content ""
    width 100%
    height 1px
    background $color
    @media screen and (-webkit-device-pixel-ratio 2)
      transform scaleY(.5)
    @media screen and (-webkit-device-pixel-ratio 3)
      transform scaleY(.333333333)

//实现背景图片对应的混合
bg-image($name)
  @media screen and (-webkit-device-pixel-ratio 2)
    background-image url($name+"@2x.png")
  @media screen and (-webkit-device-pixel-ratio 3)
    background-image url($name+"@3x.png")